The present invention relates generally to an anti-skid brake control system for an automotive hydraulic brake system. More particularly, the invention relates to a technique for measuring wheel acceleration including interporation over the interval required for deriving a wheel acceleration value on the basis of wheel speed variations.
As is well known, optimum braking characteristics are obtained when braking pressure or force can be so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. On the other hand, if wheel speed/vehicle speed ratio is maintained higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolong due to a lack of braking pressure. On the other hand, if the braking pressure is so adjusted as to maintain the wheel speed/vehicle speed to a ratio less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so that the wheel speed is held to the given optimal ratio to the vehicle speed.
In the practical anti-skid brake control operation, braking pressure in one or more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder. The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain at an optimal relationship to the vehicle speed for a satisfactorily long period of time.
U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et al discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be greater or lesser than the period of that limiting frequency above which the brake system cannot respond. In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel. Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing.
In addition, Published Japanese Patent Application (Tokkai) Showa 51-89096, published on Aug. 4, 1976 discloses a system similar to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of the increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps.
European Patent First Publications Nos. 01 23 286, 01 23 287, 01 24 035, 01 23 283, 01 23 281, 01 23 285, 01 23 280, 01 23 279, 01 23 282, 01 26 377 and 01 25 613 and German Patent First Publications Nos. 34 17 587, 34 18 217, 34 17 389, 34 17 144 and 34 18 235 disclose anti-skid brake control systems including derivation of wheel speed and wheel acceleration. In the disclosed systems, the anti-skid control system holds braking pressure at a constant level after wheel acceleration drops below a predetermined threshold.
In the recent anti-skid brake control systems disclosed in the publications listed above, an attempt has been made to maintain the braking pressure near a lock pressure at which the wheels tend to lock. A typical attempt at maintaining the braking pressure near the lock pressure has been disclosed in the European Patent First Publication No. 01 34 512. In this approach, the possibility of skidding is increased since the braking pressure can easily exceed the lock pressure briefly. In order to prevent the wheels from locking, brake pressure must be controlled more precisely. For more precise control, it is essential to detect when wheel acceleration drops below the deceleration threshold with greater accuracy.
However, in the anti-skid control systems proposed in the aforementioned publications, wheel acceleration derivation routines require much more time than in wheel speed derivation. This may cause delays in detection of when the wheel acceleration drops below the deceleration threshold which may cause the wheels to lock.